The present invention relates to bipolar electrolysis cells for the production of metal by the reduction of a metal halide in a molten bath comprising the metal halide dissolved in at least one molten halide of higher electrodecomposition potential than the metal halide. More particularly, this invention relates to such cells having a plurality of electrodes, disposed horizontally and arranged in at least one vertical stack in a superimposed, spaced relationship defining inter-electrode spaces, where metal is produced, between each pair of adjacent electrodes. The present invention relates to an improvement in such cells for collecting the metal produced while maintaining substantially constant anode-cathode spacing in the inter-electrode spaces.
Bipolar electrolysis cells for the production of metal by the reduction of a metal halide in a molten halide batch have been known for many years. A cell of this type, which is reportedly useful in connection with a process for the production of either zinc or lead by the reduction of the appropriate metal chloride, is described in U.S. Pat. No. 1,545,383 of Ashcroft. The Ashcroft cell includes a series of inclined graphite plates which function as bipolar electrodes. These electrodes are arranged in a superimposed, spaced relationship defining inter-electrode spaces between each pair of adjacent electrodes. The electrodes are enclosed within a metallic shell having an insulated refractory lining. The cell is adapted to contain a molten bath of metallic chlorides, in which the electrodes are immersed. As electrolysis proceeds in this cell, metal and chlorine are produced in the inter-electrode spaces. Because of the inclination of the electrodes, the metal produced in the cell flows, under the influence of gravity, downwardly across the cathode surfaces of the electrodes and through holes in the electrodes to a metal-collecting zone in the bottom of the cell. At the same time, the chlorine produced in the cell flows, because of its buoyancy, upwardly across the inclined anode surfaces of the electrodes and through holes therein to a gas-collecting zone in the top of the cell. The anode and cathode surfaces of the electrodes to this cell may be corrugated to facilitate the flows of metal and chlorine in the cell. These flows reportedly induce circulation of the bath in the cell and thereby facilitate continuous electrolysis. However, it has been observed that in a cell of this type, the flow of metal across the cathode surfaces may reduce the anode-cathode spacing in an uncontrolled and unpredictable fashion, and thereby reduce cell efficiency.
A bipolar electrolysis cell for the production of lead by the reduction of lead chloride in a molten halide bath is described in Bureau of Mines Report of Investigations No. 8166, entitled "Recovery of Lead From Lead Chloride by Fused-Salt Electrolysis". This cell includes a plurality of graphite plates which function as bipolar electrodes. These electrodes are arranged in a vertical stack in a superimposed, spaced relationship defining inter-electrode spaces between each pair of adjacent electrodes. The electrodes are inclined slightly from the horizontal and grooved on the anode and cathode surfaces to direct the flow of lead and chlorine produced during electrolysis to opposite sides of the cell. Gaps on both sides of the stack of electrodes allow lead to flow downwardly and chlorine to flow upwardly on opposite sides of the stack. Thus, the cell described in the Bureau of Mines Report is similar in many respects to the Ashcroft cell. In both cases, metal flows downwardly across the cathode surfaces of the electrodes, under the influence of gravity, to one side of the stack and then to the bottom of the cell, while chlorine flows upwardly across the anode surfaces of the electrodes, because of its buoyancy, to the opposite side of the stack and then to the top of the cell.
A bipolar electrolysis cell which operates in a different manner is described in U.S. Pat. No. 3,893,899 of Dell et al. This cell is particularly adapted for the production of aluminum by the electrolytic reduction of aluminum chloride in a molten halide bath. It includes an anode, at least one intermediate bipolar electrode and a cathode in a superimposed, spaced relationship defining inter-electrode spaces therebetween. These electrodes, unlike the electrodes in the Ashcroft cell, are preferably disposed horizontally within a vertical stack. Along one side of the stack of electrodes is located a bath-supply passage, which is in fluid communication with each inter-electrode space within the stack. Along the opposite side of the stack is a gas-lift passage, which is also in fluid communication with each inter-electrode space. As electrolysis proceeds in this cell, chlorine is produced on the anode surfaces of the electrodes, and metal is produced on the cathode surfaces. The chlorine is conducted through the inter-electrode spaces toward and into the gas-lift passage. This flow of chlorine induces a flow of molten bath into and out of each inter-electrode space, upwardly in the gas-lift passage, across the stack of electrodes and downwardly through the bath-supply passage. The flow of bath entrains metal produced on each cathode surface and carries it through and out of each inter-electrode space. This effectively prevents metal from accumulating on the cathode surfaces and permits desirably low anode-cathode spacing. The metal, which is entrained by the flow of bath, is carried into the gas-lift passage, where it descends under the influence of gravity, in a direction opposite to that of the rising chlorine and bath, to the bottom of the cell. However, it has been observed that in a cell of this type, under certain circumstances, metal which is carried into the gas-lift passage may there combine with the chlorine therein, or it may be carried to the top of the cell, along with the chlorine and bath, where it may combine with the chlorine to produce the metal chloride. The occurrence of these events may adversely affect the efficiency of the cell, since a portion of the metal reduced combines with chlorine to produce the metal chloride.
Bipolar electrolysis cells have also been used to produce aluminum or another metal by the reduction of the metal oxide in a molten halide bath. Because of the nature of the bath used in such cells, electrolytically active, carbonaceous cathode surfaces in these cells are often protected from attack by bath constituents by a layer of molten metal. Such cells are usually operated so that metal produced therein accumulates in a protective layer on the cathode surfaces of the electrodes in the cell. Such a method of operation, while advantageously protecting the cathode surfaces, may reduce cell efficiency by continuously building up a layer of metal on the cathode surfaces, thereby continuously varying the anode-cathode spacing.
A bipolar electrolysis cell which is used to produce aluminum by the reduction of aluminum oxide in a molten halide bath is described in U.S. Pat. No. 2,959,533 of de Varda. FIG. 2 of this patent shows a cell having a plurality of inclined bipolar electrodes which are spaced so as to form inter-electrode spaces between each pair of adjacent electrodes. The cathode surface of each electrode includes a number of cavities which are adapted to retain a portion of the molten aluminum produced in the cell. When filled with molten aluminum, these cavities form electrolytically active, protected areas on the cathode surfaces. Aluminum produced in the inter-electrode spaces in excess of that required to fill these cavities flows down the inclined cathode surfaces into a metal-collecting zone in the bottom of the cell. However, such a flow of metal across the cathode surfaces of a cell of this type may change the anode-cathode spacing in an uncontrolled and unpredictable manner, and thereby reduce cell power efficiency.
Another bipolar electrolysis cell of the type which is adapted to retain molten metal in a protective layer on its cathode surfaces is described in U.S. Pat. No. 3,554,893 of de Varda. This cell includes a plurality of bipolar electrodes which are disposed horizontally in a vertical stack with inter-electrode spaces between each pair of adjacent electrodes. Each of these electrodes is in the form of a flat tray with upwardly curved borders. The concave upper cathode face of each electrode fills with metal during electrolysis. Metal produced thereafter overflows from the cathode surface and drops onto the surface of the electrode below or onto the bottom of the cell. However, in this type of construction, the metal may overflow the cathode surface at a variety of different and possibly nonrepeating places. In a cell designed for directional flow of evolved gas and circulating bath constituents, as well as reduced metal, such directed flow of metal could cause problems such as inducing recombination of reduced metal with halogen gas should the spillover of metal be directly in the path of gas flow.